TLS resumption: restrict session re-use

12 files changed, 148 insertions, 72 deletions

diff --git a/doc/doc-txt/ChangeLog b/doc/doc-txt/ChangeLog
index 239731436..8c46dcc7d 100644
--- a/doc/doc-txt/ChangeLog
+++ b/doc/doc-txt/ChangeLog
@@ -126,6 +126,11 @@ JH/28 OpenSSL: fix transport-required OCSP stapling verification under session
       passed on the wire for the restarted session. Fix by using the recorded
       ocsp status of the stored session for the new connection.
 
+JH/29 TLS resumption: the key for session lookup in the client now includes
+      more info that a server could potentially use in configuring a TLS
+      session, avoiding oferring mismatching sessions to such a server.
+      Previously only the server IP was used.
+
 
 Exim version 4.95
 -----------------
diff --git a/src/exim_monitor/em_hdr.h b/src/exim_monitor/em_hdr.h
index ee05815df..24146d3a7 100644
--- a/src/exim_monitor/em_hdr.h
+++ b/src/exim_monitor/em_hdr.h
@@ -95,7 +95,7 @@ this interface so that this kind of kludge isn't needed. */
 #ifndef NS_MAXMSG
 # define NS_MAXMSG 65535
 #endif
-typedef void hctx;
+typedef void * hctx;
 
 #include "local_scan.h"
 #include "macros.h"
diff --git a/src/src/auths/dovecot.c b/src/src/auths/dovecot.c
index 3331cb856..ca3d1bd1c 100644
--- a/src/src/auths/dovecot.c
+++ b/src/src/auths/dovecot.c
@@ -275,11 +275,20 @@ if (cctx.sock < 0)
 # ifndef DISABLE_TLS
 if (ob->server_tls)
   {
-  uschar * s;
+  union sockaddr_46 interface_sock;
+  EXIM_SOCKLEN_T size = sizeof(interface_sock);
   smtp_connect_args conn_args = { .host = &host };
-  tls_support tls_dummy = {.sni=NULL};
+  tls_support tls_dummy = { .sni = NULL };
   uschar * errstr;
 
+  if (getsockname(cctx->sock, (struct sockaddr *) &interface_sock, &size) == 0)
+    conn_args.sending_ip_address = host_ntoa(-1, &interface_sock, NULL, NULL);
+  else
+    {
+    *errmsg = string_sprintf("getsockname failed: %s", strerror(errno));
+    goto bad;
+    }
+
   if (!tls_client_start(&cctx, &conn_args, NULL, &tls_dummy, &errstr))
     {
     auth_defer_msg = string_sprintf("TLS connect failed: %s", errstr);
diff --git a/src/src/exim.h b/src/src/exim.h
index 2541baa3d..9d1819677 100644
--- a/src/src/exim.h
+++ b/src/src/exim.h
@@ -535,8 +535,8 @@ config.h, mytypes.h, and store.h, so we don't need to mention them explicitly.
 #include "hintsdb_structs.h"
 #include "structs.h"
 #include "blob.h"
-#include "globals.h"
 #include "hash.h"
+#include "globals.h"
 #include "functions.h"
 #include "dbfunctions.h"
 #include "osfunctions.h"
diff --git a/src/src/globals.h b/src/src/globals.h
index 8a6405b47..e8635eefc 100644
--- a/src/src/globals.h
+++ b/src/src/globals.h
@@ -108,6 +108,9 @@ typedef struct {
     OCSP_VFIED			/* verified */
     }     ocsp;		      /* Stapled OCSP status */
 #ifndef DISABLE_TLS_RESUME
+  hctx	  resume_hctx;		/* session lookup key accumulation */
+  const uschar * resume_index;	/* session lookup key */
+
   unsigned resumption;		/* Session resumption */
   BOOL	  host_resumable:1;
   BOOL	  ticket_received:1;
diff --git a/src/src/host.c b/src/src/host.c
index e99e6ceba..2b09cc260 100644
--- a/src/src/host.c
+++ b/src/src/host.c
@@ -914,14 +914,14 @@ if (type < 0)
     struct sockaddr_in6 *sk = (struct sockaddr_in6 *)arg;
     yield = US inet_ntop(family, &(sk->sin6_addr), CS addr_buffer,
       sizeof(addr_buffer));
-    if (portptr != NULL) *portptr = ntohs(sk->sin6_port);
+    if (portptr) *portptr = ntohs(sk->sin6_port);
     }
   else
     {
     struct sockaddr_in *sk = (struct sockaddr_in *)arg;
     yield = US inet_ntop(family, &(sk->sin_addr), CS addr_buffer,
       sizeof(addr_buffer));
-    if (portptr != NULL) *portptr = ntohs(sk->sin_port);
+    if (portptr) *portptr = ntohs(sk->sin_port);
     }
   }
 else
@@ -940,7 +940,7 @@ if (Ustrncmp(yield, "::ffff:", 7) == 0) yield += 7;
 if (type < 0)
   {
   yield = US inet_ntoa(((struct sockaddr_in *)arg)->sin_addr);
-  if (portptr != NULL) *portptr = ntohs(((struct sockaddr_in *)arg)->sin_port);
+  if (portptr) *portptr = ntohs(((struct sockaddr_in *)arg)->sin_port);
   }
 else
   yield = US inet_ntoa(*((struct in_addr *)arg));
diff --git a/src/src/lookups/readsock.c b/src/src/lookups/readsock.c
index dfde99945..bb1e6ca9a 100644
--- a/src/src/lookups/readsock.c
+++ b/src/src/lookups/readsock.c
@@ -116,10 +116,20 @@ else
 #ifndef DISABLE_TLS
 if (do_tls)
   {
+  union sockaddr_46 interface_sock;
+  EXIM_SOCKLEN_T size = sizeof(interface_sock);
   smtp_connect_args conn_args = {.host = &host };
-  tls_support tls_dummy = {.sni=NULL};
+  tls_support tls_dummy = { .sni = NULL };
   uschar * errstr;
 
+  if (getsockname(cctx->sock, (struct sockaddr *) &interface_sock, &size) == 0)
+    conn_args.sending_ip_address = host_ntoa(-1, &interface_sock, NULL, NULL);
+  else
+    {
+    *errmsg = string_sprintf("getsockname failed: %s", strerror(errno));
+    goto bad;
+    }
+
   if (!tls_client_start(cctx, &conn_args, NULL, &tls_dummy, &errstr))
     {
     *errmsg = string_sprintf("TLS connect failed: %s", errstr);
diff --git a/src/src/structs.h b/src/src/structs.h
index 46cc99ff6..9bf3aebe2 100644
--- a/src/src/structs.h
+++ b/src/src/structs.h
@@ -830,6 +830,7 @@ typedef struct {
   host_item *           host;
   int                   host_af;
   uschar *              interface;
+  uschar *		sending_ip_address;	/* used for TLS resumption */
   int			sock;	/* used for a bound but not connected socket */
 
 #ifdef SUPPORT_DANE
diff --git a/src/src/tls-gnu.c b/src/src/tls-gnu.c
index 622782369..634d4011e 100644
--- a/src/src/tls-gnu.c
+++ b/src/src/tls-gnu.c
@@ -2867,12 +2867,12 @@ NULL plist return for silent no-ALPN.
 */
 
 static BOOL
-tls_alpn_plist(const uschar * tls_alpn, const gnutls_datum_t ** plist, unsigned * plen,
+tls_alpn_plist(uschar ** tls_alpn, const gnutls_datum_t ** plist, unsigned * plen,
   uschar ** errstr)
 {
 uschar * exp_alpn;
 
-if (!expand_check(tls_alpn, US"tls_alpn", &exp_alpn, errstr))
+if (!expand_check(*tls_alpn, US"tls_alpn", &exp_alpn, errstr))
   return FALSE;
 
 if (!exp_alpn)
@@ -2902,11 +2902,12 @@ return TRUE;
 static void
 tls_server_set_acceptable_alpns(exim_gnutls_state_st * state, uschar ** errstr)
 {
+uschar * local_alpn = string_copy(tls_alpn);
 int rc;
 const gnutls_datum_t * plist;
 unsigned plen;
 
-if (tls_alpn_plist(tls_alpn, &plist, &plen, errstr) && plist)
+if (tls_alpn_plist(&local_alpn, &plist, &plen, errstr) && plist)
   {
   /* This seems to be only mandatory if the client sends an ALPN extension;
   not trying ALPN is ok. Need to decide how to support server-side must-alpn. */
@@ -3268,25 +3269,25 @@ however avoid storing and retrieving session information. */
 
 static void
 tls_retrieve_session(tls_support * tlsp, gnutls_session_t session,
-  host_item * host, smtp_transport_options_block * ob)
+  smtp_connect_args * conn_args, smtp_transport_options_block * ob)
 {
 tlsp->resumption = RESUME_SUPPORTED;
-if (verify_check_given_host(CUSS &ob->tls_resumption_hosts, host) == OK)
+if (verify_check_given_host(CUSS &ob->tls_resumption_hosts, conn_args->host) == OK)
   {
   dbdata_tls_session * dt;
   int len, rc;
   open_db dbblock, * dbm_file;
 
-  DEBUG(D_tls)
-    debug_printf("check for resumable session for %s\n", host->address);
   tlsp->host_resumable = TRUE;
+  tls_client_resmption_key(tlsp, conn_args, ob);
+
   tlsp->resumption |= RESUME_CLIENT_REQUESTED;
   if ((dbm_file = dbfn_open(US"tls", O_RDONLY, &dbblock, FALSE, FALSE)))
     {
-    /* Key for the db is the IP.  We'd like to filter the retrieved session
-    for ticket advisory expiry, but 3.6.1 seems to give no access to that */
+    /* We'd like to filter the retrieved session for ticket advisory expiry,
+    but 3.6.1 seems to give no access to that */
 
-    if ((dt = dbfn_read_with_length(dbm_file, host->address, &len)))
+    if ((dt = dbfn_read_with_length(dbm_file, tlsp->resume_index, &len)))
       if (!(rc = gnutls_session_set_data(session,
 		    CUS dt->session, (size_t)len - sizeof(dbdata_tls_session))))
 	{
@@ -3332,8 +3333,7 @@ if (gnutls_session_get_flags(session) & GNUTLS_SFLAGS_SESSION_TICKET)
       if ((dbm_file = dbfn_open(US"tls", O_RDWR, &dbblock, FALSE, FALSE)))
 	{
 	/* key for the db is the IP */
-	dbfn_delete(dbm_file, host->address);
-	dbfn_write(dbm_file, host->address, dt, dlen);
+	dbfn_write(dbm_file, tlsp->resume_index, dt, dlen);
 	dbfn_close(dbm_file);
 
 	DEBUG(D_tls)
@@ -3368,14 +3368,14 @@ return 0;
 
 static void
 tls_client_resume_prehandshake(exim_gnutls_state_st * state,
-  tls_support * tlsp, host_item * host,
+  tls_support * tlsp, smtp_connect_args * conn_args,
   smtp_transport_options_block * ob)
 {
 gnutls_session_set_ptr(state->session, state);
 gnutls_handshake_set_hook_function(state->session,
   GNUTLS_HANDSHAKE_NEW_SESSION_TICKET, GNUTLS_HOOK_POST, tls_client_ticket_cb);
 
-tls_retrieve_session(tlsp, state->session, host, ob);
+tls_retrieve_session(tlsp, state->session, conn_args, ob);
 }
 
 static void
@@ -3473,7 +3473,7 @@ if (ob->tls_alpn)
   const gnutls_datum_t * plist;
   unsigned plen;
 
-  if (!tls_alpn_plist(ob->tls_alpn, &plist, &plen, errstr))
+  if (!tls_alpn_plist(&ob->tls_alpn, &plist, &plen, errstr))
     return FALSE;
   if (plist)
     if (gnutls_alpn_set_protocols(state->session, plist, plen, 0) != 0)
@@ -3565,7 +3565,7 @@ if (request_ocsp)
 #endif
 
 #ifdef EXIM_HAVE_TLS_RESUME
-tls_client_resume_prehandshake(state, tlsp, host, ob);
+tls_client_resume_prehandshake(state, tlsp, conn_args, ob);
 #endif
 
 #ifndef DISABLE_EVENT
diff --git a/src/src/tls-openssl.c b/src/src/tls-openssl.c
index ab3b636a3..bab02d056 100644
--- a/src/src/tls-openssl.c
+++ b/src/src/tls-openssl.c
@@ -3620,21 +3620,21 @@ return DEFER;
 and apply it to the ssl-connection for attempted resumption. */
 
 static void
-tls_retrieve_session(tls_support * tlsp, SSL * ssl, const uschar * key)
+tls_retrieve_session(tls_support * tlsp, SSL * ssl)
 {
-tlsp->resumption |= RESUME_SUPPORTED;
 if (tlsp->host_resumable)
   {
+  const uschar * key = tlsp->resume_index;
   dbdata_tls_session * dt;
   int len;
   open_db dbblock, * dbm_file;
 
   tlsp->resumption |= RESUME_CLIENT_REQUESTED;
-  DEBUG(D_tls) debug_printf("checking for resumable session for %s\n", key);
+  DEBUG(D_tls)
+    debug_printf("checking for resumable session for %s\n", tlsp->resume_index);
   if ((dbm_file = dbfn_open(US"tls", O_RDWR, &dbblock, FALSE, FALSE)))
     {
-    /* key for the db is the IP */
-    if ((dt = dbfn_read_with_length(dbm_file, key, &len)))
+    if ((dt = dbfn_read_with_length(dbm_file, tlsp->resume_index, &len)))
       {
       SSL_SESSION * ss = NULL;
       const uschar * sess_asn1 = dt->session;
@@ -3660,7 +3660,7 @@ if (tlsp->host_resumable)
 	if (lifetime + dt->time_stamp < time(NULL))
 	  {
 	  DEBUG(D_tls) debug_printf("session expired\n");
-	  dbfn_delete(dbm_file, key);
+	  dbfn_delete(dbm_file, tlsp->resume_index);
 	  }
 	else if (SSL_set_session(ssl, ss))
 	  {
@@ -3716,9 +3716,7 @@ if (SSL_SESSION_is_resumable(ss)) 	/* 1.1.1 */
 
   if ((dbm_file = dbfn_open(US"tls", O_RDWR, &dbblock, FALSE, FALSE)))
     {
-    const uschar * key = cbinfo->host->address;
-    dbfn_delete(dbm_file, key);
-    dbfn_write(dbm_file, key, dt, dlen);
+    dbfn_write(dbm_file, tlsp->resume_index, dt, dlen);
     dbfn_close(dbm_file);
     DEBUG(D_tls) debug_printf("wrote session (len %u) to db\n",
 		  (unsigned)dlen);
@@ -3728,21 +3726,20 @@ return 1;
 }
 
 
+/* Construct a key for session DB lookup, and setup the SSL_CTX for resumption */
+
 static void
 tls_client_ctx_resume_prehandshake(
-  exim_openssl_client_tls_ctx * exim_client_ctx, tls_support * tlsp,
-  smtp_transport_options_block * ob, host_item * host)
+  exim_openssl_client_tls_ctx * exim_client_ctx, smtp_connect_args * conn_args,
+  tls_support * tlsp, smtp_transport_options_block * ob)
 {
-/* Should the client request a session resumption ticket? */
-if (verify_check_given_host(CUSS &ob->tls_resumption_hosts, host) == OK)
-  {
-  tlsp->host_resumable = TRUE;
+tlsp->host_resumable = TRUE;
+tls_client_resmption_key(tlsp, conn_args, ob);
 
-  SSL_CTX_set_session_cache_mode(exim_client_ctx->ctx,
-	SSL_SESS_CACHE_CLIENT
-	| SSL_SESS_CACHE_NO_INTERNAL | SSL_SESS_CACHE_NO_AUTO_CLEAR);
-  SSL_CTX_sess_set_new_cb(exim_client_ctx->ctx, tls_save_session_cb);
-  }
+SSL_CTX_set_session_cache_mode(exim_client_ctx->ctx,
+      SSL_SESS_CACHE_CLIENT
+      | SSL_SESS_CACHE_NO_INTERNAL | SSL_SESS_CACHE_NO_AUTO_CLEAR);
+SSL_CTX_sess_set_new_cb(exim_client_ctx->ctx, tls_save_session_cb);
 }
 
 static BOOL
@@ -3766,7 +3763,7 @@ if (tlsp->host_resumable)
 
 tlsp->resumption = RESUME_SUPPORTED;
 /* Pick up a previous session, saved on an old ticket */
-tls_retrieve_session(tlsp, ssl, host->address);
+tls_retrieve_session(tlsp, ssl);
 return TRUE;
 }
 
@@ -3786,16 +3783,19 @@ if (SSL_session_reused(exim_client_ctx->ssl))
 #ifdef EXIM_HAVE_ALPN
 /* Expand and convert an Exim list to an ALPN list.  False return for fail.
 NULL plist return for silent no-ALPN.
+
+Overwite the passed-in list with the expanded version.
 */
 
 static BOOL
-tls_alpn_plist(const uschar * tls_alpn, const uschar ** plist, unsigned * plen,
+tls_alpn_plist(uschar ** tls_alpn, const uschar ** plist, unsigned * plen,
   uschar ** errstr)
 {
 uschar * exp_alpn;
 
-if (!expand_check(tls_alpn, US"tls_alpn", &exp_alpn, errstr))
+if (!expand_check(*tls_alpn, US"tls_alpn", &exp_alpn, errstr))
   return FALSE;
+*tls_alpn = exp_alpn;
 
 if (!exp_alpn)
   {
@@ -3976,39 +3976,20 @@ if (tls_client_basic_ctx_init(exim_client_ctx->ctx, host, ob,
       client_static_state, errstr) != OK)
   return FALSE;
 
-#ifndef DISABLE_TLS_RESUME
-tls_client_ctx_resume_prehandshake(exim_client_ctx, tlsp, ob, host);
-#endif
-
-
-if (!(exim_client_ctx->ssl = SSL_new(exim_client_ctx->ctx)))
-  {
-  tls_error(US"SSL_new", host, NULL, errstr);
-  return FALSE;
-  }
-SSL_set_session_id_context(exim_client_ctx->ssl, sid_ctx, Ustrlen(sid_ctx));
-
-SSL_set_fd(exim_client_ctx->ssl, cctx->sock);
-SSL_set_connect_state(exim_client_ctx->ssl);
-
 if (ob->tls_sni)
   {
   if (!expand_check(ob->tls_sni, US"tls_sni", &tlsp->sni, errstr))
     return FALSE;
   if (!tlsp->sni)
-    {
-    DEBUG(D_tls) debug_printf("Setting TLS SNI forced to fail, not sending\n");
-    }
+    { DEBUG(D_tls) debug_printf("Setting TLS SNI forced to fail, not sending\n"); }
   else if (!Ustrlen(tlsp->sni))
     tlsp->sni = NULL;
   else
     {
-#ifdef EXIM_HAVE_OPENSSL_TLSEXT
-    DEBUG(D_tls) debug_printf("Setting TLS SNI \"%s\"\n", tlsp->sni);
-    SSL_set_tlsext_host_name(exim_client_ctx->ssl, tlsp->sni);
-#else
+#ifndef EXIM_HAVE_OPENSSL_TLSEXT
     log_write(0, LOG_MAIN, "SNI unusable with this OpenSSL library version; ignoring \"%s\"\n",
           tlsp->sni);
+    tlsp->sni = NULL;
 #endif
     }
   }
@@ -4019,10 +4000,10 @@ if (ob->tls_alpn)
   const uschar * plist;
   unsigned plen;
 
-  if (!tls_alpn_plist(ob->tls_alpn, &plist, &plen, errstr))
+  if (!tls_alpn_plist(&ob->tls_alpn, &plist, &plen, errstr))
     return FALSE;
   if (plist)
-    if (SSL_set_alpn_protos(exim_client_ctx->ssl, plist, plen) != 0)
+    if (SSL_CTX_set_alpn_protos(exim_client_ctx->ctx, plist, plen) != 0)
       {
       tls_error(US"alpn init", host, NULL, errstr);
       return FALSE;
@@ -4035,6 +4016,29 @@ if (ob->tls_alpn)
           ob->tls_alpn);
 #endif
 
+#ifndef DISABLE_TLS_RESUME
+if (verify_check_given_host(CUSS &ob->tls_resumption_hosts, host) == OK)
+  tls_client_ctx_resume_prehandshake(exim_client_ctx, conn_args, tlsp, ob);
+#endif
+
+
+if (!(exim_client_ctx->ssl = SSL_new(exim_client_ctx->ctx)))
+  {
+  tls_error(US"SSL_new", host, NULL, errstr);
+  return FALSE;
+  }
+SSL_set_session_id_context(exim_client_ctx->ssl, sid_ctx, Ustrlen(sid_ctx));
+SSL_set_fd(exim_client_ctx->ssl, cctx->sock);
+SSL_set_connect_state(exim_client_ctx->ssl);
+
+#ifdef EXIM_HAVE_OPENSSL_TLSEXT
+if (tlsp->sni)
+  {
+  DEBUG(D_tls) debug_printf("Setting TLS SNI \"%s\"\n", tlsp->sni);
+  SSL_set_tlsext_host_name(exim_client_ctx->ssl, tlsp->sni);
+  }
+#endif
+
 #ifdef SUPPORT_DANE
 if (conn_args->dane)
   if (dane_tlsa_load(exim_client_ctx->ssl, host, &conn_args->tlsa_dnsa, errstr) != OK)
diff --git a/src/src/tls.c b/src/src/tls.c
index bc3261ad2..a988c7505 100644
--- a/src/src/tls.c
+++ b/src/src/tls.c
@@ -3,7 +3,7 @@
 *************************************************/
 
 /* Copyright (c) University of Cambridge 1995 - 2018 */
-/* Copyright (c) The Exim Maintainers 2020 - 2021 */
+/* Copyright (c) The Exim Maintainers 2020 - 2022 */
 /* See the file NOTICE for conditions of use and distribution. */
 
 /* This module provides TLS (aka SSL) support for Exim. The code for OpenSSL is
@@ -25,6 +25,11 @@ functions from the OpenSSL or GNU TLS libraries. */
 #endif
 
 
+/* Forward decl. */
+static void tls_client_resmption_key(tls_support *, smtp_connect_args *,
+  smtp_transport_options_block *);
+
+
 #if defined(MACRO_PREDEF) && !defined(DISABLE_TLS)
 # include "macro_predef.h"
 # ifdef USE_GNUTLS
@@ -791,6 +796,44 @@ return status == 0;
 
 
 
+static void
+tls_client_resmption_key(tls_support * tlsp, smtp_connect_args * conn_args,
+  smtp_transport_options_block * ob)
+{
+hctx * h = &tlsp->resume_hctx;
+blob b;
+gstring * g;
+
+#ifdef EXIM_HAVE_SHA2
+exim_sha_init(h, HASH_SHA2_256);
+#else
+exim_sha_init(h, HASH_SHA1);
+#endif
+
+//  TODO: word from server EHLO resp		/* how, fer gossakes?  Add item to conn_args or tls_support? */
+
+if (conn_args->dane)
+  exim_sha_update(h, CUS &conn_args->tlsa_dnsa, sizeof(dns_answer));
+exim_sha_update(h,   conn_args->host->address,	Ustrlen(conn_args->host->address));
+exim_sha_update(h,   CUS &conn_args->host->port, sizeof(conn_args->host->port));
+exim_sha_update(h,   conn_args->sending_ip_address, Ustrlen(conn_args->sending_ip_address));
+if (openssl_options)
+  exim_sha_update(h, openssl_options,		Ustrlen(openssl_options));
+if (ob->tls_require_ciphers)
+  exim_sha_update(h, ob->tls_require_ciphers,	Ustrlen(ob->tls_require_ciphers));
+if (tlsp->sni)
+  exim_sha_update(h, tlsp->sni,			Ustrlen(tlsp->sni));
+#ifdef EXIM_HAVE_ALPN
+if (ob->tls_alpn)
+  exim_sha_update(h, ob->tls_alpn,		Ustrlen(ob->tls_alpn));
+#endif
+exim_sha_finish(h, &b);
+for (g = string_get(b.len*2+1); b.len-- > 0; )
+  g = string_fmt_append(g, "%02x", *b.data++);
+tlsp->resume_index = string_from_gstring(g);
+DEBUG(D_tls) debug_printf("TLS: resume session index %s\n", tlsp->resume_index);
+}
+
 #endif	/*!DISABLE_TLS*/
 #endif	/*!MACRO_PREDEF*/
 
diff --git a/src/src/transports/smtp.c b/src/src/transports/smtp.c
index 2f718a1e4..f9e319c79 100644
--- a/src/src/transports/smtp.c
+++ b/src/src/transports/smtp.c
@@ -2665,6 +2665,7 @@ if (  smtp_peer_options & OPTION_TLS
   else
   TLS_NEGOTIATE:
     {
+    sx->conn_args.sending_ip_address = sending_ip_address;
     if (!tls_client_start(&sx->cctx, &sx->conn_args, sx->addrlist, &tls_out, &tls_errstr))
       {
       /* TLS negotiation failed; give an error. From outside, this function may